The invention relates to an MR apparatus which includes
a cylindrical main field magnet for generating a uniform, steady magnetic field whose cylindrical shape defines a symmetry axis,
a gradient coil system which encloses the symmetry axis and includes a plurality of gradient coils,
an RF coil system which is situated inside the gradient coil system and includes a plurality of mutually offset conductors which extend parallel to the symmetry axis, and
a table top for accommodating an object to be examined.
The invention also relates to a combination of a gradient coil system and an RF coil system which is suitable for use in an MR apparatus.
The electric power required for feeding a gradient coil is dependent not only on the speed at which the current through the gradient coil is switched and off, but also on the diameter of the coil; that is, the necessary electric power increases approximately as the fifth power of the diameter. Therefore, the inner diameter of the gradient coil system generally is chosen to be so large that the RF shield which is situated directly on the inner side of the gradient coil system and serves to shield the RF coil from the environment is situated at an adequate distance therefrom. If the inner diameter of the gradient coil system, and hence the distance between the RF shield situated therein and the RF coil system, were reduced, the electric power required for feeding the gradient coils would be further reduced, but the power required for feeding the RF coil would be increased. The gradient coil customarily has a circular inner contour which matches the shape of the RF coil whose conductors which extend parallel to the symmetry axis are arranged on a circle around the symmetry axis.
EP-OS 313 213 discloses a gradient coil system in which at least one of the gradient coils is assembled from a plurality of circular and elliptical segments in such a manner that the dimensions of the inner diameter of these coils (14xe2x80x3) in the vertical direction are smaller than those in the horizontal direction. Inside this gradient coil there can still be arranged an RF coil for examinations of the skull which has a diameter of 12xe2x80x3. It is also stated that one (or more) of the coils may be assembled from a combination of circular, elliptical or otherwise shaped segments.
It is an object of the present invention to provide an MR apparatus in which the electric power required for feeding the gradient coil system can be further reduced. This object is achieved according to the invention in that the conductors of the RF coil system which are situated underneath the table top are situated at a distance from the symmetry axis which is smaller than the distance between the symmetry axis and the conductors situated above the table top, and that the cross-section of the gradient coil system is adapted to the cross-section of the RF coil system.
The invention utilizes the fact that the space underneath the table top of the MR apparatus is not available for positioning the patient anyway. Therefore, at this area the conductors of the RF coil system as well as the gradient coil system are situated nearer to the symmetry axis in comparison with the area above the table top. As a result, the power required to feed the gradient coil is reduced in comparison with the power required for an arrangement with a circular cross-section of the RF coil system and the gradient coil system.
For the manufacture of the gradient coil system offers advantages in comparison with a gradient coil system having a circular cross-section.
The embodiment enables a reduction of the electric power required for gradient coils whose magnetic field has a gradient whose direction deviates from the direction of the symmetry axis. Moreover, the spatial homogeneity of the field generated by the RF coil is improved in the direction of the symmetry axis. This is because, the RF magnetic field exhibits a pronounced maximum at its center in the case of a (circular) cylindrical shape of the gradient coil system. This maximum is strongly flattened or widened because at this area the gradient coil, and hence also the RF shield situated on its inner side, is situated at the smallest distance from the RF coil. A further advantage of this arrangement consists in that the so-called patient opening in the MR apparatus becomes wider towards the outside, thus facilitating the access and mitigating the patient""s feelings of being locked up.
For a predetermined cross-section of a gradient coil system the isocenter thereof (being the point at which all magnetic fields generated by the gradient coils assume the value zero) can be chosen at will within given limits if the turns of the individual gradient coils extend suitably in space. When the isocenter is situated above the symmetry axis embodiments of the gradient coil system are obtained which are particularly attractive with a view to power requirements.
The magnetic fields produced by the gradient coil system may cause eddy currents in the metal housing in which the main field magnet is accommodated; such eddy currents lead to field distortions. The occurrence of these eddy currents is customarily suppressed by means of active shield which encloses the gradient coil system and includes the same number of coils as the gradient coil system. When these coils are shaped as a circular cylinder whose central axis is coincident with the central axis, in the embodiment of the gradient coils the magnetic field generated in the outer region by the gradient coils will be stronger at the top than at the bottom, so that it could occur that at the top and/or the bottom the magnetic fields of the coils of the shield and the gradient coils can no longer compensate one another.
However, when the density of the turns of the coils of the active shield above the symmetry axis is higher than that therebelow, the magnetic field generated by this coil will also be larger at the top than at the bottom, so that the magnetic field generated in its interior by the corresponding gradient coils is attenuated more above the symmetry axis than below this axis.